The present invention relates to an improved breathable web and an economical method for advantageously modifying the physical properties of a web for use as a component of a disposable absorbent article, and to disposable articles incorporating such modified webs.
Disposable, wearable articles having an inner, body-facing, liquid pervious component, an intermediate, absorbent, liquid-retaining component and an outer, garment-facing, liquid-impervious component are well known. Articles of that type are commonly available in the form of disposable diapers, disposable underwear, pull-on diapers and training pants, incontinence pads, incontinence briefs, sanitary napkins, pantiliners, and the like. Such articles generally include a flexible, liquid-impervious outercover (i.e., backsheet) that is adapted to be positioned between an absorbent component of the article and the clothing of the wearer, to prevent wetting or soiling of the wearer""s clothing when the article is in use.
In order to provide improved comfort to the wearer of disposable absorbent articles, certain components of the articles, such as a backsheet, in addition to providing imperviousness to liquids, desirably permit the passage therethrough of moisture vapor and also preferably air, to help maintain dryness and to reduce the humidity adjacent the wearer""s body. An impervious polymeric film to which breathability has been imparted to allow air and moisture vapor transmission through the film is disclosed U.S. Pat. No. 3,156,342, entitled xe2x80x9cFlexible Absorbent Sheet,xe2x80x9d which issued on Nov. 10, 1964, to G. A. Crowe, Jr.; U.S. Pat. No. 3,881,489, entitled xe2x80x9cBreathable, Liquid Impervious Backsheet for Absorbent Devices,xe2x80x9d which issued on May 6, 1975 to Edward Wallace Hartwell, et al.; U.S. Pat. No. 3,989, 867, entitled xe2x80x9cAbsorbent Devices Having Porous Backsheet,xe2x80x9d which issued on Nov. 2, 1976, to James Bryant Sisson; U.S. Pat. No. 4,153,751, entitled xe2x80x9cProcess for Stretching an Impregnated Film of Material and The Microporous Product Produced Thereby,xe2x80x9d which issued on May 8, 1979, to Eckhard C. A. Schwarz; and U.S. Pat. No. 4,539,256, entitled xe2x80x9cMicroporous Sheet Material, Method of Making and Articles Made Therewith,xe2x80x9d which issued on Sep. 3, 1985, to Gene H. Shipman.
In addition to imperviousness to liquids, and pervious to moisture vapor and air, the backsheet also preferably includes a cloth-like outer surface, which provides a softer feel, and also a more appealing visual appearance, as compared with the outer surface of a smooth, flat plastic film. Two-ply backsheets that provide a desirable, more cloth-like appearance for such disposable, wearable articles are also known. In that regard, U.S. Pat. No. 5,151,092, entitled xe2x80x9cAbsorbent Article with Dynamic Elastic Waist Feature Having a Predisposed Resilient Flexural Hinge,xe2x80x9d which issued on Sep. 29, 1992, to Kenneth B. Buell, et al., discloses a disposable diaper backsheet formed either from a woven or a nonwoven material, a polymeric film, or a composite material in the form of a film-coated, nonwoven material. That patent also discloses the step of embossing of a plastic film backsheet to provide a more cloth-like appearance to a plastic film.
Also known to those skilled in the art are methods for imparting extensibility to an otherwise substantially inelastic material, which may be employed as a backsheet. For example, the use of corrugating rolls to laterally or longitudinally stretch and to simultaneously provide a corrugated form to thin plastic films is disclosed in U.S. Pat. No. 4,116,892, entitled xe2x80x9cProcess for Stretching Incremental Portions of an Orientable Thermoplastic Substrate and Product Thereof,xe2x80x9d which issued on Sep. 26, 1978, to Eckhard C. A. Schwarz; U.S. Pat. No. 4,834,741, entitled xe2x80x9cDiaper With Waistband Elastic,xe2x80x9d which issued on May 30, 1989, to Reinhardt N. Sabee; U.S. Pat. No. 5,156,793, entitled xe2x80x9cMethod for Incrementally Stretching Zero Strain Stretch Laminate Sheet In A Non-Uniform manner To Impart A Varying Degree Of Elasticity Thereto,xe2x80x9d which issued on Oct. 20, 1992, to Kenneth B. Buell et al.; U.S. Pat. No. 5,167,897, entitled xe2x80x9cMethod for Incrementally Stretching A Zero Strain Stretch Laminate Sheet To Impart Elasticity Thereto,xe2x80x9d which issued on Dec. 1, 1992 to Gerald M. Webber et al.; and U.S. Pat. No. 5,422,172, entitled xe2x80x9cElastic Laminated Sheet of An Incrementally Stretched Nonwoven Fibrous Sheet and Elastomeric Film and Method,xe2x80x9d which issued on Jun. 6, 1995, to Pai-Chuan Wu. The corrugating rolls disclosed in each of those patents are employed in carrying out a process sometimes referred to as xe2x80x9cring-rolling,xe2x80x9d to locally stretch and form corrugations in the material, in order to impart a greater degree of stretchability to selected portions of a sheet or web that may serve as a backsheet for disposable absorbent articles. Such backsheets can include both a polymeric film and an overlying and contacting layer of nonwoven, fibrous material.
Although there have been significant product improvements in recent years that have resulted in improved functioning and increased consumer acceptance of disposable absorbent articles, it is still desirable to provide an improved material having optimal physical properties relating to permeability to water vapor and air, and impermeability to liquid. Additionally, the improved material should desirably have the optimal properties that are particularly useful in an absorbent article, such as good liquid impact value, and air flow rate. It is further desirable that the improved material has a soft, cloth-like outer surface and extensibility useful for comfort and fit provided by absorbent articles containing such a material.
It would also be advantageous to provide an economical method for modifying a pre-formed web or laminate to have desirable properties which can be used as a structural component or an extensible component of a disposable absorbent article.
The present invention provides an improved method of modifying the physical characteristics of a web at high web speed. The precursor web is passed between at least one pair of interengaged rolls to incrementally stretch the web, then the incrementally stretched web is withdrawn from between the rollers under tension. An engineering strain rate from about 50 sxe2x88x921 to about 1650 sxe2x88x921 is found to be effective in achieving a modified web with desired properties. The method of the present invention is applicable at a temperature from ambient to about 120xc2x0 C., and a web speed from about 30 m/min to about 365 m/min.
The modified web has a microporous structure containing few large pores and capillaries. Thus, the resultant microporous web is breathable to air or vapor, but acts as a liquid barrier under the impact pressure commonly imposed by the wearer of an absorbent article. Particularly, the microporous web has good breathability (as manifested in moisture vapor transmission rate and air flow rate) while maintaining satisfactorily low leakage under impact pressure. Additionally, the modified web can have a soft, cloth-like surface and extensibility for improved fit and wearer comfort.
The webs may be films, nonwovens, or composites of films and nonwoven webs, such as laminates. Typically, the web is made of thermoplastic materials which may be a blend of thermoplastic polymers and pore-forming agents such as incompatible organic materials or inorganic particulate materials. In the incremental stretching process of the present invention, the pore-forming agent is activated and the resultant web has pores and channels (i.e., interconnected pores) through the thickness of its structure, which is generally referred to as a xe2x80x9cmicroporousxe2x80x9d web.
The breathable and liquid impermeable web of the present invention comprises at least a microporous film and has an MVTR (moisture vapor transmission rate) of at least about 2000 g/m2/24 hr, a dynamic impact value of less than about 10 g/ m2, a bubble pressure at least about 45 psi, and optionally, an air flow of at least about 2 liters/m2/s.